The present invention relates to the communication of signals via optical fibers, and more particularly to a wavelength selective optical coupler that is useful in combining a plurality of optical signals for communication via a common transmission path and in providing a high power optical fiber amplifier.
Cable television systems currently distribute television program signals via coaxial cable, typically arranged in tree and branch networks. Coaxial cable distribution systems require a large number of high bandwidth electrical amplifiers. For example, forty or so amplifiers may be required between the cable system headend and an individual subscriber's home.
The use of a television signal comprising amplitude modulated vestigial sideband video subcarriers (AM-VSB) is preferred in the distribution of cable television signals due to the compatibility of that format with the standards of the National Television Systems Committee (NTSC) and the ability to provide an increased number of channels within a given bandwidth. An undesirable characteristic of AM-VSB transmission, however, is that it requires a much higher carrier-to-noise ratio (CNR) than other techniques, such as frequency modulation or digital transmission of video signals. Generally, a CNR of at least 40 dB is necessary to provide clear reception of AM-VSB television signals.
The replacement of coaxial cable with optical fiber transmission lines in television distribution systems has become a high priority. Production single mode fiber can support virtually unlimited bandwidth and has low attenuation. Accordingly, a fiber optic distribution system or a fiber-coax cable hybrid would provide substantially increased performance at a competitive cost as compared to prior art coaxial cable systems. In order to implement such systems, optical couplers are necessary to couple different optical signals to the distribution network or to components of the distribution network, such as optical amplifiers.
Amplification of optical signals within an optical fiber network has been a problem in the attempt to distribute AM-VSB television signals. As noted above, amplifiers are required between a cable system headend and a subscriber's home in order to provide signals to the subscriber at an acceptable power level. Semiconductor optical amplifiers of the type typically used in fiber optic systems produce high levels of distortion products that are not compatible with multi-channel AM-VSB video signals. This is due to the short lifetime of the carrier excited state within the semiconductor optical amplifier. The recombination time of such an amplifier operating near 1.3 .mu.m or 1.5 .mu.m is about 1.2 nanoseconds, which is short compared to the period of a typical AM-VSB subcarrier operating in the television band of about 55.25 MHz to 1 GHz.
Optical fiber amplifiers, such as erbium-doped fiber amplifiers, have been proposed for applications in long distance transmission and subscriber loop distribution systems. See, e.g., W. I. Way, et al, "Noise Figure of a Gain-Saturated Erbium-Doped Fiber Amplifier Pumped at 980 nm", Optical Amplifiers and Their Applications, 1990 Technical Digest Series, Vol. 13, Conference Edition, Optical Society of America, August 6-8, 1990, Paper TuB3, pp. 134-137, and C. R. Giles, "Propagation of Signal and Noise in Concatenated Erbium-Doped Fiber Optical Amplifiers", Journal of Lightwave Technology, Vol. 9, No. Feb. 2, 1991, pp. 147-154.
The noise figure of the fiber amplifier is a parameter that must be considered in such systems to optimize overall system performance. Noise figures of an erbium-doped fiber amplifier pumped at 980 nm have been found to be near 3 dB, which is a desirable performance figure. However, an erbium-doped fiber amplifier pumped at 980 nm does not exhibit an optimal power efficiency for a communication signal distributed at a typical wavelength of about 1550 nm.
In order to provide a higher power efficiency for a 1550 nm communication signal, erbium-doped fiber amplifiers can be pumped at about 1480 nm. However, pumping at this wavelength results in a noise figure of about 5 dB, which is less than optimal.
One way to increase the power efficiency of a rare earth fiber amplifier, such as an erbium fiber amplifier, is to increase the pump power to the doped fiber. High power pump lasers suitable for use with rare earth fiber amplifiers, and particularly erbium fiber amplifiers, have not been readily available at a low enough cost for wide scale use in cable television distribution systems. It would therefore be advantageous to provide a scheme for providing high power pumping energy at relatively low cost. It would be further advantageous to provide an improved technique for coupling pump energy to a fiber amplifier using a simple and inexpensive optical component, such as a grating.
Recent progress has been made in placing gratings in optical fibers by modifying the fiber index of refraction. Examples of processes for forming such gratings can be found in G. Meltz, W. W. Morey and W. H. Glenn, Optical Letters, Vol. 14, p. 823, 1989 and R. Kashgap, J. R. Armitage, R. Wyatt, S. T. Davey, and D. L. Williams, Electronics Letters. Vol. 26, p. 730, 1990. These articles describe the formation of gratings by photorefractive techniques. Fiber gratings can also be fabricated according to the teachings of C. M. Ragdale, et al, "Bragg Grating Add-Drop Optical Multiplexers for InP Based Optoelectronic Integrated Circuits," Integrated Photonics Research Conference, IEEE OSA Meeting, Apr. 9, 1991, Monterey, California, Paper TuD12.
The gratings disclosed in the articles cited above are perpendicular to the direction in which the optical signal propagates through the substrate containing the grating. When the grating is placed perpendicular to the direction of the lightwave propagation, the light is reflected back upon its original path. Thus, such gratings are used as reflectors.
In the field of optical communications, it is desirable to multiplex different optical signals onto a single optical fiber. The optical fiber can then distribute the various signals for selective retrieval at a receiver. It is also desirable to provide low cost, high power lasers for use in communicating signals via optical fibers. As an alternative or complement to a high power source laser, the provision of a high power optical fiber amplifier is desirable.
In order to achieve the above, it would be advantageous to provide a wavelength selective optical fiber coupler. It would be further advantageous to provide a high power optical fiber amplifier that can make effective use of a wavelength selective optical fiber coupler to provide a low cost solution for the amplification of optical signals in a communication network, such as a cable television distribution network. It would be still further advantageous to provide a wavelength selective optical coupler that can be used to multiplex a plurality of different optical information signals onto a common transmission path.
The present invention provides a wavelength selective optical coupler that enjoys the aforementioned advantages. A high power optical amplifier and an optical multiplexing scheme embodying the coupler are also provided.